where kwd is a keywordp. The effect of this event is to modify the
meaning of the :dir keyword argument of include-book as indicated
by the examples above, namely by removing association of any directory with
the indicated keyword for purposes of the include-book:dir
argument. Normally one would instead use add-include-book-dir to
associate a new directory with that keyword; see add-include-book-dir.

Note: This is an event! It does not print the usual event summary
but nevertheless changes the ACL2 logical world and is so recorded.